Probe molecules, acidic dissociation constant

Acid centers, structure, sulfate-supported metal oxides, 37 192-196 Acidic catalysis, 6 241 montmorillonite, 38 266-268 Acidic dissociation constant, probe molecules, 38 210... 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

Proton Affinities, Vertical Ionization Potentials, and Acidic Dissociation Constants of Selected Basic Probe Molecules"... 

However, acidity or basicity of a gas-phase adsorbate is not a sole criterion for its choice as a probe molecule. Firstly, the strength of an acidic or basic probe should be distinguished accordingly to its acid- or base-dissociation constant Ka or Kb). In addition, very important feature of probe molecule is its radius. If there is a need to locate all active sites in the structure of microporous solid material, the radius of probe molecule has to be smaller then the diameter of pore(s) opening(s). In other words, probe molecules have to be of appropriate size, so the entrance in the micropores of the solid and the access of adsorbate to each active site become possible. For example, ammonia, which is frequently used to reveal the acidic property of solids, is selected as a probe due to its basicity and due to the size of the molecule. Its molecule is smaller than the diameter of the pores in the zeolites structures, and also in many other solids. The other probe often used for investigation of solids acidity is pyridine however, the application of other chemical species is also possible. 

Thus pH = — log(cH,b). while cb and chb represent the concentrations of the dissociated and protonated species of the acid-hase chromophore pair, respectively. The quantity pK — F< s/(RTln 10) can be considered as an apparent pK for proton binding of the chromophoric acid-base couple on the membrane surface. Eqn. 5.8 shows that the protonatiou state Cb/chb of the probe is altered if s changes at constant pH, which results in a change in the fluorescence yield. Thus changes of fluorescence at constant bulk pH reflect chauges of aud can be utilised, therefore, to study how molecules interact with membranes. 

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